Process for preparing (meth) acrylic anhydride

ABSTRACT

The invention relates to a process for the batchwise preparation of (meth)acrylic anhydride, in which acetic anhydride is reacted with (meth)acrylic acid and the acetic acid is at least partly removed gradually as it is formed.  
     This process is characterized in that the acetic acid removed is at least partly replaced by introducing into the reaction medium, during the reaction, acetic anhydride and/or (meth)acrylic acid.  
     The (meth)acrylic anhydride obtained by this process may be used in the synthesis of (meth)acrylic thioesters, (meth)acrylic amides and (meth)acrylic esters, in polymerization reactions or as crosslinking agents.

[0001] The present invention relates to a process for the batchwisepreparation of (meth)acrylic anhydride.

[0002] Acrylic and methacrylic anhydrides are reagents of choice in thethe synthesis of (meth)acrylic thioesters, (meth)acrylic amides and(meth)acrylic esters, in particular of tertiary alcohols that cannot beobtained by standard esterification/transesterification processes.

[0003] They are also used in polymerization reactions or as crosslinkingagents.

[0004] It has been known for a long time that it is possible to preparean anhydride by reacting acetic anhydride with the acid corresponding tothe desired anhydride.

[0005] As early as 1934, French patent No. 784 458 described thepreparation of propanoic, butyric and caproic anhydride by reactingacetic anhydride with propanoic acid, butyric acid and caproic acid,respectively.

[0006] In 1979, European patent application No. 4 641 provided thepublic with a continuous or batchwise process for preparing carboxylicacid anhydrides such as benzoic, hexahydrobenzoic and trimethylaceticanhydride, by reacting, in reaction/distillation apparatus, thecorresponding acids with acetic anhydride, preferably in stoichiometricproportions.

[0007] In 1986, German patent application No. 3 510 035 disclosed aprocess for continuously preparing carboxylic acid anhydrides such asacrylic or methacrylic anhydride by reacting, in a distillation columnand in the presence of a catalyst such as sulphuric acid or sulphonic orphosphoric acids, acetic anhydride with the acid corresponding to thedesired anhydride.

[0008] In 1987, French patent application No. 2 592 040 proposed aprocess for the batchwise synthesis of (meth)acrylic anhydride byreacting acetic anhydride with (meth)acrylic acid in the presence ofpolymerization inhibitors. According to this process, acetic anhydrideand (meth)acrylic acid are first reacted together, the acetic acidformed during the reaction is withdrawn and a distillation is thencarried out. The molar ratio between the (meth)acrylic acid and theacetic acid is chosen between 0.5 and 5 and preferably between 2 and2.2.

[0009] However, the implementation of this process comes up againstpolymerization problems. In addition, the amount of anhydride producedis limited by the size of the reactor and thus by the amount of reagentsloaded into this reactor.

[0010] The aim of the present invention is thus to propose a process forpreparing (meth)acrylic anhydride which offers higher productionefficiency and a reduction or even elimination of the risks ofpolymerization.

[0011] One subject of the invention is thus a process for the batchwisepreparation of (meth)acrylic anhydride, in which acetic anhydride isreacted with (meth)acrylic acid and at least some of the acetic acid isremoved gradually as it is formed.

[0012] This process is characterized in that the acetic acid removed isat least partly replaced by introducing into the reaction medium, duringthe reaction, acetic anhydride and/or (meth)acrylic acid.

[0013] Such a process allows a greater than 35% increase in productionefficiency compared with the prior art processes.

[0014] Other characteristics and advantages of the invention will now bedescribed in detail in the description that follows.

DETAILED DESCRIPTION OF THE INVENTION

[0015] The invention is based on the surprising discovery made by theApplicant, namely that, by means of an astute distribution of thereagents, it is possible to obtain large gains in production per batch(batchwise) of (meth)acrylic anhydride, without increasing the reactiontime.

[0016] This very large increase in production per batch is obtained withan identical initial mass of reagents (acetic anhydride and(meth)acrylic acid).

[0017] This initial charge is preferably the maximum charge permitted bythe volume of the reactor.

[0018] Thus, according to the invention, the acetic acid, which isformed by reaction of the acetic anhydride and which is at least partlyremoved gradually as it is formed, is replaced with one and/or the otherof the reagents.

[0019] In other words, an amount of the reagent(s) is added to occupythe space liberated by the removal of the acetic acid.

[0020] Preferably, all the acetic acid that is formed during thereaction is removed, gradually and as it is formed, by distillation.

[0021] In order to optimize the production of (meth)acrylic anhydride,it is desirable to replace, by means of the reagent(s), all the aceticacid removed.

[0022] In addition, a continuous addition of the reagent(s) throughoutthe reaction time is a variant that is preferable to an irregularaddition.

[0023] It is also preferable that this addition should follow, asclosely as possible, the removal of the acetic acid.

[0024] This is then reflected by a virtually total occupation of thevolume of the reactor throughout the reaction.

[0025] Preferably, only one of the reagents is added.

[0026] The reagent added is advantageously acetic anhydride.

[0027] Furthermore, the initial charge introduced into the reactorpreferably has an initial molar ratio R₀ of the (meth)acrylic acid tothe acetic anhydride of between 2.5 and 11 and in particular between 9and 11.

[0028] The overall molar ratio R_(g) of the (meth)acrylic acid to theacetic anhydride is preferably between 0.5 and 5 and in particularbetween 1.8 and 2.2.

[0029] The reaction may be carried out in a reactor on which is mounteda distillation column.

[0030] In general, the reactor is stirred and heated by circulatingheat-exchange fluid in a jacket or by recirculation through an externalheat exchanger.

[0031] The distillation column preferably has a separating efficiency ofgreater than 10 theoretical plates and in particular greater than 12theoretical plates. This makes it possible to minimize the losses ofacetic anhydride via the first distillation fraction, which in this caseconsists to more than 99% of acetic acid, to work at low levels ofreflux (R/C less than or equal to 2/1) and consequently to reduce thereaction time and the risks of polymerization that increase as thereaction time increases.

[0032] The column packing may be a standard packing, in bulk form orstructured, or a mixture of these two types of packing.

[0033] The reaction temperature is generally between 50 and 120° C. andpreferentially between 85 and 105° C.

[0034] The pressure is adjusted as a function of the chosen reactiontemperature. In general, it is between 20 and 200 mm Hg (0.0267 and0.2666 bar).

[0035] The reaction may be carried out in “isobar” mode, i.e. by fixingthe pressure and allowing the temperature to change up to a limit valuepreferably fixed between 90 and 150° C., or in “isothermal” mode, i.e.by fixing the temperature and adjusting the pressure in the plantthroughout the reaction so as to maintain this pressure.

[0036] The temperature at the column head is advantageously adjustedduring the reaction, as a function of the pressure, so as to correspondto the distillation temperature of acetic acid.

[0037] By working in this way, a head fraction containing more than 99%acetic acid is obtained.

[0038] According to one preferred embodiment of the invention, thereaction between acetic anhydride and (meth)acrylic acid is carried outin the presence of at least one polymerization inhibitor.

[0039] In addition, a double-stabilization is preferably carried out, byintroducing at least one inhibitor into the reactor and at least oneinhibitor into the distillation column.

[0040] The inhibitors must be active with respect to polymerizationwhile at the same time being inert with regard to the anhydrides and the(meth)acrylic acid.

[0041] Thus, all risk of polymerization in the reactor and the column isavoided.

[0042] The inhibitor for the reactor is advantageously chosen from thegroup consisting of 2,4-dimethyl-6-tert-butylphenol (“Topanol A”) and2,6-di-tert-butyl-para-cresol (“BHT”), and mixtures thereof.

[0043] The inhibitor for the distillation column is advantageouslychosen from the group consisting of hydroquinone (“HQ”),2,4-dimethyl-6-tert-butylphenol, 2,6-di-tert-butyl-para-cresol andphenothiazine, and mixtures thereof.

[0044] As regards the amounts to be used, the reactor inhibitor ispreferably introduced into the initial charge of reagents in aproportion of at least 0.001% (1000 ppm) by weight of the charge.

[0045] The distillation column inhibitor is preferably introduced intothe distillation column throughout the reaction, for example as a 5%solution (by weight relative to the total weight of the solution) inacetic acid. The flowrate of introduction of the column inhibitor isadjusted so as to have less than 1000 ppm of inhibitor in the finalreactor product.

[0046] Sparging with depleted air (8% oxygen and 92% nitrogen by volume)may be carried out throughout the reaction.

[0047] The crude product obtained is generally perfectly clear, free ofpolymers and able to be freed of the head fraction by distillation underreduced pressure (for example of 20 mm Hg) so as to rid it of the excessacetic acid, of (meth)acrylic acid and of the compound formed byreaction of 1 mole of (meth)acrylic acid with 1 mole of aceticanhydride.

[0048] The process according to the invention may comprise a furtherstep of distillation of the crude product obtained, where appropriateafter removal of the head fraction, on a distillation column or using ashort-residence-time machine such as a film evaporator.

EXAMPLES

[0049] The examples that follow illustrate the present inventionwithout, however, limiting its scope. The percentages therein areexpressed on a mass basis.

[0050] The following abbreviations are used therein:

[0051] MAA: methacrylic acid

[0052] AA: acrylic acid

[0053] MA₂OA: methacrylic anhydride

[0054] A₂OA: acrylic anhydride

[0055] Ac₂O: acetic anhydride

[0056] AcOH: acetic acid

[0057] Mixed: H₂C═CHCOOOCCH₃, or, depending on the case,

H₂C═C(CH₃)COOOCCH₃

Example 1 (Comparative)

[0058] 361 g (3.54 mol) of Ac₂O and 639 g (7.43 mol) of MAA (the molarratio R₀ is thus 2.1) are introduced into a mechanically stirred reactorheated by circulation of thermostatically-maintained oil in a jacket,and on which is mounted a distillation column containing Multiknit®structured packing having a separation efficacy of 12 theoreticalplates, and being able to function under vacuum.

[0059] 1.09 g of Topanol A are introduced as inhibitor into the reactor,and a solution of 5% Topanol A and 5% HQ in acetic acid is introduced asinhibitor into the column, this solution being added uniformlythroughout the duration of the reaction at a rate of 2 ml/h.

[0060] Sparging with depleted air (8% oxygen and 92% nitrogen by volume)is maintained in the reactor throughout the operation.

[0061] The acetic acid formed is removed gradually as it is formed. Thefirst fraction distilled off is composed of 99.5% acetic acid.

[0062] After reaction for 6 hours 30 minutes at 95° C., the crudereaction product has the following composition: AcOH 0.5% Ac₂O 0.06% MAA11.8% Mixed 4.6% MA₂OA 81.6% Side products 1.44% TOTAL 100%

[0063] The amount of MA₂OA contained in the crude product (measured byweighing at the end of the reaction) is 485 g.

[0064] Thus, the degree of conversion of Ac₂O into MA₂OA and Mixed is96.5%.

[0065] The final molar ratio R_(f) MAA/Ac₂O is 2.1; it is identical tothe initial molar ratio.

[0066] The crude product obtained is then freed of the head fractionunder reduced pressure (20 mm Hg) in order to remove therefrom theresidual MAA and the Mixed.

[0067] The crude product obtained after removal of the head fractionconsists of 96.2% MA₂OA.

Example 2 (According to the Invention)

[0068] The process is performed as indicated in Example 1, except thatthe same mass (1000 g) of overall initial charge of MAA and of Ac₂O isadded to the reactor, but with more MAA and less Ac₂O being introduced.

[0069] The initial charge thus consists of 900 g (10.465 mol) of MAA and100 g (0.98 mol) of Ac₂O (the initial molar ratio R₀ is 10.7).

[0070] The same inhibitor as in Example 1 is introduced in the sameproportion into the reactor.

[0071] The same inhibitor solution as in Example 1 is introduced at thesame flowrate into the column.

[0072] Ac₂O is introduced uniformly throughout the reaction, graduallyas the AcOH is removed, so as to occupy all the space liberated by thesaid AcOH in the reactor.

[0073] Working in this way allows optimum occupation of the reactionvolume.

[0074] After reaction for 6 hours 30 minutes at 95° C., the amount ofAc₂O introduced continuously during the reaction was 408 g and the crudereaction product has the following composition: AcOH 0.4% Ac₂O 0.2% MAA12.3% Mixed 7.2% MA₂OA 78.8% Side products 1.44% TOTAL 100%

[0075] The amount of MA₂OA contained in the crude product (measured byweighing at the end of reaction) is 680 g, for a total amount of reagentused of 1408 g (1000+408).

[0076] The final molar ratio R_(f) is 2.1, as in Example 1.

[0077] Consequently, the gain in production of MA₂OA is 40% relative toExample 1, without increasing the reaction time and with the same finalmolar ratio R_(f).

[0078] Thus, for the same reaction volume (same initial mass of 1000 g),but with a better distribution of the reagents, much more MA₂OA wasproduced.

[0079] The degree of conversion of Ac₂O into MA₂OA and Mixed is 97%.

[0080] The crude product obtained after removal of the head fraction isentirely clear, free of polymers and able to be distilled under reducedpressure (20 mm Hg). It consists of 96.4% MA₂OA.

Example 3 (Comparative) and Example 4 (According to the Invention)

[0081] The process as indicated in Examples 1 and 2 is performed,replacing the methacrylic acid with acrylic acid and using adistillation column having an efficacy of 20 theoretical plates.

[0082] All the other conditions are identical to those of Examples 1 and2.

[0083] At the end of the 6 hours 30 minutes of reaction at 95° C., thefollowing table may be produced: Example 4 Example 3 (according to theSynthesis A₂OA (comparative) invention) AA charged at the 535 735 start(g) Ac₂O charged at the 361 161 start (g) Total charged at the 896 896start (g) Ac₂O introduced 0 335 continuously during the reaction Totalcharged (g) 896 1231 throughout the operation A₂OA produced (g) 360 497

[0084] The gain in production for Example 4 relative to Example 3 isthus 38%.

1. Process for the batchwise preparation of (meth)acrylic anhydride, inwhich acetic anhydride is reacted with (meth)acrylic acid and at leastsome of the acetic acid is removed gradually as it is formed,characterized in that the acetic acid removed is at least partlyreplaced by introducing into the reaction medium, during the reaction,acetic anhydride and/or (meth)acrylic acid.
 2. Process according toclaim 1, in which the reagent introduced is acetic anhydride.
 3. Processaccording to claim 2, in which the initial molar ratio R₀ of the(meth)acrylic acid to the acetic anhydride is between 2.5 and 11 andpreferably between 9 and
 11. 4. Process according to claim 2, in whichthe overall molar ratio of the (meth)acrylic acid to the aceticanhydride is between 0.5 and 5 and preferably between 1.8 and 2.2. 5.Process according to claim 3, in which the overall molar ratio of the(meth)acrylic acid to the acetic anhydride is between 0.5 and 5 andpreferably between 1.8 and 2.2.
 6. Process according to claim 1, inwhich all the acetic acid which is formed during the reaction is removedgradually as it is formed.
 7. Process according to claim 2, in which allthe acetic acid which is formed during the reaction is removed graduallyas it is formed.
 8. Process according to claim 3, in which all theacetic acid which is formed during the reaction is removed gradually asit is formed.
 9. Process according to claim 1, in which all the aceticacid removed is replaced with (meth)acrylic anhydride and/or(meth)acrylic acid.
 10. Process according to claim 2, in which all theacetic acid removed is replaced with (meth)acrylic anhydride and/or(meth)acrylic acid.
 11. Process according to claim 3, in which all theacetic acid removed is replaced with (meth)acrylic anhydride and/or(meth)acrylic acid.
 12. Process according to claim 4, in which all theacetic acid removed is replaced with (meth)acrylic anhydride and/or(meth)acrylic acid.
 13. Process according to claim 1, in which theintroduction of acetic anhydride and/or of (meth)acrylic acid isperformed continuously throughout the reaction.
 14. Process according toclaim 1, in which the reaction is carried out in a reactor on which ismounted a distillation column.
 15. Process according to claim 14, inwhich the distillation column has a separation efficacy of greater than10 theoretical plates and preferably greater than 12 theoretical plates.16. Process according to one of claim 1, in which the reaction betweenthe acetic anhydride and the (meth)acrylic acid is carried out in thepresence of at least one polymerization inhibitor.
 17. Process accordingto claim 1, in which the reaction is carried out in a reactor on whichis mounted a distillation column and in which at least one inhibitor isintroduced into the reactor and at least one inhibitor is introducedinto the distillation column.
 18. Process according to claim 17, inwhich: the reactor inhibitor is chosen from the group consisting of2,4-dimethyl-6-tert-butylphenol and 2,6-di-tert-butyl-para-cresol, andthe mixtures thereof; and the distillation column inhibitor is chosenfrom the group consisting of hydroquinone,2,4-dimethyl-6-tert-butylphenol, 2,6-di-tert-butyl-para-cresol andphenothiazine, and mixtures thereof.
 19. Process according to claim 18,in which: the reactor inhibitor is introduced into the initial charge ofreagents, in a proportion of at least 0.001% by weight; the distillationcolumn inhibitor is introduced into the distillation column throughoutthe reaction.
 20. Use of a (meth)acrylic anhydride obtained by theprocess according to claim 1 in the synthesis of (meth)acrylicthioesters, (meth)acrylic amides and (meth)acrylic esters, in particularof tertiary alcohols, in polymerization reactions or as crosslinkingagents.